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Your thoughts on IRS

Balr14

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Way back in the "old days", we built a few early 30s street rods and used the IRS from a Jaguar. They fit well, looked cool and made a hell of a difference to ride and handling.
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Norm Peterson

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Pretty sure lap times at tracks around the world disagree with this.
I'm pretty sure that all of those videos showing big bushing deformations were taken during dragstrip runs where harsh launching and harsh upshifting is part of the game (and easy advertising to those who don't look past the picture part).


Norm
 

Norm Peterson

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How can someone even say a stick is better than the IRS? Anyone that thinks different should come follow me on the elevated sections of the interstate in my city. My 12 loved to try to kill me on that. My old 15 and 18 value my life.
Even I'm not saying that a stick axle is better than an IRS. Not as long as the IRS has been properly thought out, which was not the case in ponycars until within the last decade.

What I will say is that a stick axle isn't as bad as so many people make it out to be (an attitude coming in part just because IRS is the newest and therefore has to be better in all respects).

Stick axle driving probably does require more careful driving technique and maybe a butt-dyno that's more sensitive to tires progressing from linear through transition to pure friction based grip, and for the differences in roll stiffness balance as the car rolls in a turn.

Which is probably at odds with what today's nannies even let you experience up where you start needing that experience. In my lifetime, that's been dead-nuts easy to come by, so I don't even have to think about how to deal with it. In today's nannified driving world, or for drivers less sensitive to such things that's most likely not the case. No flame intended.


Norm
 
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Sivi70980

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I'm pretty sure that all of those videos showing big bushing deformations were taken during dragstrip runs where harsh launching and harsh upshifting is part of the game (and easy advertising to those who don't look past the picture part).


Norm
Dragstrips and tracks are different though. I'm 100% sure you know better than I do about what works better around corners on a track. My own experience shows the S197 GT500 was a little scary. The Mini JCW was the opposite kind of scary aka "holy cow amazing!". The S550 actually feels close to the mini on the streets at least.
 

Norm Peterson

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I think it's worth adding that the same anti-squat 'hit' which drag racers are able to use to their advantage with drag slicks tends to hurt braking and corner-exit power down on roadrace cars. (Regardless of what Herb Adams wrote ~30 years ago in his book)

Many moons ago, Trans Am (the race series) allowed both live and independent rear axle cars. It worked until the IRS guys figured things out & dominated. At that point, they outlawed the IRS.
I don't think Herb Adams' book went deep enough into the anti-squat discussion, and was generally oriented to torque arm suspensions as used on the 3rd/4th gen F-body cars.

What I get with torque arm suspensions is that anti-squat drops off fairly rapidly as rear ride height drops below static ride height. But that's probably outside his target audience.

If you're not aware of it, there's at least one current-ish thread over on TMO that's getting into anti-squat. Mostly from a 3-link/PHB perspective, but there's some general information like when you're in a corner the inboard and outboard a-s values might not be the same, and that the shape of the a-s curve would then have influence of its own since the two wheels would be operating at slightly different points.


Norm
 

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Norm Peterson

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Perhaps I'm a few minutes behind, but the difference between the current IRS and the 03-04 Cobra IRS is night-and-day different, I mean, 15+ years development changes things an awful lot. The 10R80 is 15+ years better than the AOD-E, just as the GT500's TVS R2650 is better than the 03-04's (h)Eaton M112.
Absolutely.

For those who weren't there (let alone earlier), the idea that a well-thought-out stick axle suspension could be better than an indifferently-designed IRS was true. All except for rough road ride needs.


Anecdotal, to be sure, but by my third track day on NJMP/Lightning I was turning lap times comparable to stockish 2010 SS Camaro times. With over 100 HP less, smaller tires, stick axle vs IRS, and three longer stretches of essentially straight track, totaling about 1 mile of WOT acceleration out of 1.9 miles total lap distance.


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Norm Peterson

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Dragstrips and tracks are different though. I'm 100% sure you know better than I do about what works better around corners on a track. My own experience shows the S197 GT500 was a little scary.
I'm not surprised. Weight, weight distribution, tires, tire sixes, and wheel widths at least were conspiring against it (especially in the 2008-ish years).

I think you'd have liked my 2008 GT much better. Less understeer than in later years, and surprisingly competent during a 65+ mph slalom through a field of truck tire debris even when new.

The Mini JCW was the opposite kind of scary aka "holy cow amazing!". The S550 actually feels close to the mini on the streets at least.
I kind of get what you're saying. IRS does have a smoother feel to it, which makes it feel easier to drive. In daily driving (meaning not when I'm "testing") I drive our WRX just as hard through the curves as I would the Mustang, and the WRX does have a somewhat 'lighter' feel. More so after a little alignment and tire pressure tinkering.

Somewhere there's a Mustang driver I made an inside pass in the WRX against where he thought he was going to gap me through and out of this two-lane curve instead (video was snipped from a Mustang test run). He was trying, hard.




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TeeLew

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I don't think Herb Adams' book went deep enough into the anti-squat discussion, and was generally oriented to torque arm suspensions as used on the 3rd/4th gen F-body cars.

Norm
If you care, you can look again. He addresses several live and independent rear configurations. He has a page of calculations that show anti-squat producing 'extra' normal load on the rear tires which is over and above that of static load and dynamic load transfer. It's some sort of magical force which give 'extra' grip to the rear of the car. The truth is that there's not 'extra' normal load and any 'anti' is simply a component of dynamic load transfer. On a road race car, the more we use, the more throttle-on problems the rear tends to have. He is critical of independent suspensions because of their relative lack of anti-squat. He calls this, "...a distinct disadvantage." It's not.

After read (and believing) his book, I spent time on my Fox-body to add anti-squat with the available aftermarket parts of the day. I only succeeded in making the rear of the car sketchy as hell with wheel-hop on the brakes and a distinct lack of on-throttle rear grip. The car would pirouette at the slightest provocation. The good part is that this car helped me to appreciate the advantages of understeer and the importance of being able to put power down on corner exit. You often learn more by doing it wrong than by doing it right.
 

Norm Peterson

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If you want to feel the REAL difference between an IRS and SRA car, hit some small potholes while going through a corner. SRA cars unsettle like crazy in those instances. Obviously, there's more benefits than just simply that, but that's an easily felt difference on the street. Bottom line though, I'm not sure how you wouldn't feel the difference in every day driving unless you really just don't do any spirited driving at all. The later S197 cars had a lot of suspension improvements in the SRA department, and it should be distinguished that the SRA in your 1st gen mustang and the SRA in the 5th gen are VERY VERY different.

Personally, I own both a 2017 GT and a 1965 Coupe, and there is absolutely no contest. You can make SRA handle well on a road course; the American Iron series, nascar, etc have show us this much. But SRA will always be inherently bad on rough and uneven surfaces.
Precisely. A stick-axle car may not be bothered much by running up on inside curbings at the track (inboard wheels being lightly loaded) but you will feel it more going over any gators that sing to you on corner exit.


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Norm Peterson

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If you care, you can look again. He addresses several live and independent rear configurations. He has a page of calculations that show anti-squat producing 'extra' normal load on the rear tires which is over and above that of static load and dynamic load transfer. It's some sort of magical force which give 'extra' grip to the rear of the car. The truth is that there's not 'extra' normal load and any 'anti' is simply a component of dynamic load transfer. On a road race car, the more we use, the more throttle-on problems the rear tends to have. He is critical of independent suspensions because of their relative lack of anti-squat. He calls this, "...a distinct disadvantage." It's not.
Back when he wrote that book, IRS cars in general suffered from low values of anti-squat. Corvettes included. Get on the throttle just a little in a lower gear and feel the tail head for the pavement. He was old-school GM and either didn't see what was to come, didn't want to believe it, or wasn't going to confuse his readers with it.

That "magical force" would be from anti-squat values above 100%, which corresponds to the "separation" that drag racers talk about. What happens there is that the geometric resistance to dynamic rearward load transfer temporarily exceeds the actual amount of rearward load transfer that is happening. So the rear of the car actually rises up. The payback being, of course, when the separation cannot be maintained, all the "magic" part of rear bite goes away and the car comes back down. You might even get "delayed wheelspin".


After read (and believing) his book, I spent time on my Fox-body to add anti-squat with the available aftermarket parts of the day. I only succeeded in making the rear of the car sketchy as hell with wheel-hop on the brakes and a distinct lack of on-throttle rear grip. The car would pirouette at the slightest provocation. The good part is that this car helped me to appreciate the advantages of understeer and the importance of being able to put power down on corner exit. You often learn more by doing it wrong than by doing it right.
No surprises there . . .


Norm
 

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Norm Peterson

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If you care, you can look again. He addresses several live and independent rear configurations. He has a page of calculations that show anti-squat producing 'extra' normal load on the rear tires which is over and above that of static load and dynamic load transfer.
I have been running my own lateral and rearward load transfer calculations for years, breaking them down into component loads.

On a time history basis, geometric load transfer develops first - almost instantaneously. Last of the load transfer mechanisms to fully develop is the basic force equilibrium equation coming from forward (or lateral) acceleration (it's final-position sensitive). In between comes the effect of dampers, which are linked to suspension velocity which peaks before position does.

All of that gets added algebraically to the car's static rear weight.

A word on the force equilibrium component - that's only looking at horizontal effects. Full equilibrium has to include induced vertical displacements.


I lightly edited a couple of my earlier posts (and marked them up).


Norm
 

308 Cal. Bullitt

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That's all reasonable. Let me guess...I'm the only one to ever ask?
Well. Yeah. In an <8.8- Super situation> .....very few guys ask... % wise...

Everyone who needs those #'d, low ratio's..
...generally turbo situations, % wise....,
Who want the <numerical> basement ratio #s.. dont use Super 8.8s to begin w/.

(Us Screaming Screw Blowers Idiots spinning Norms hollow laminated rotors, @22k+ rpm.. We want <numerically>
'St. Louis Arch #sized gears,..)

In our situation, & most guys, would NEVER be runnin the 8.8", or a 8.8 Super w/big pwr. 🤷

No doubt, we DO hav tricks to keep the
(Gm 12 bolt copy, & its beefed up big bro) 8.8 ALIVE under duress.
We learned most of it long ago, on the 12 bolt GM.

Example; die hard GM guys wouldn't put a 9" in there chit. Mopar guys--diehard Dana dudes, cuz Chrysler used um in muscle production cars. So its acceptable, but a 9" wasnt. It contaminated the DNA of there vehicles by there own subconscious stupidity standards.

Im runnin a Yugo Rearend, if its the best fir my application in a competitive situation. (no ideawhen that scenarioappliesother than a lawn tractor ... maybe??)

Bottom line.
Any rearend with ONLY a 1.500" vertical pinion drop,
10 bolt GM
12 bolt GM
orig 8.8/+ SUPER
14 bolt GM Truck <w/3rd bearing support like a 9", & still has a rear removable cover,>
ect....
The 1.500" distance below, the Ring gear Center line, will only hold so much pwr over so many hits. But U will all argue. Yeah, but takes less hp to turn.

Ummmm... yeah.. y'all jus keep swallowin Ron 1Jeremy's jizz on that idea under real pwr situations

Reason is muti faceted. But a good gear hobb guy can cut much more profit a day on 1.5" stuff, than the 1957-1986, 9" production rear end design w/a 2.25" vert pinion drop.

Even the famed Dana 60 has an easy to cut hypoid angle, & yet has weakness due to its own lack of distance on its vert pin drop.

All that horseshit bout
* less hp to turn *
a 10/12 bolt gm/8.8" vs a 9".
Its a Great physics-based analogy..

yet people once believed, that Matador Bulls chased Red flags more,
or that Dogs sweat by panting,
or that Earth was once, flat..well not once anymore.. idiots teaching kids this flat Earth chit. Morons.

All like the HP loss thing, on diff rearends.
Its not the HP thing that matters at the end of the day.

Its what produces the qwikest and fastest
"A to B" time.
That time IS NOT = horse Power gains or losses... at both a rudimentary level, or at a Nasa/F1 level. Load a gear, it tries to separate. A gear that takes more hp to turn on paper, may also take less under load, & usually does, cuz its difficult to calc the Shock and Awe loads of dropping the hammer on 1500hp from a standing start w/a 17" wide tire, 800 ft lbs of Torq x a 2.89 low gear x a 5.43 ring gear...

so ur math says u got 12,500lbft goin into ur gear set. Now calc the resistance
(Infinitesimally amt if variables at the grounds tractive force adhesion)
based on 100's of factors, on a drag strip starting line.... start w/track temp as #1 of 100+. 😆


If u really wanna know the entire napin based answer. I will do it.
But its long.
And includes previously documented situations of the biggest Differential builder's in the US racing industry, & $millions in WASTE'd time R&D, to build an exact copy of a 9", but just at 1.5" vert drop.
Like the low hp it takes to turn 8.8/12 bolt.
Its all Great on paper. Great on FEA.
Disaster on the track if ur trying to win races.
Not prove the soldid math of a physics calc.

Ole man 'Jeg' made Pearman
(crew chief) redo the entire driveline angle for there (NHRA ProStk cars) w/MW's & Dewco's
"New, Low Drag , High pinion, 9"",
which was only tenths of a degree diff, from ~1.63° norm,
Yet everything else related to the drive/crank/trans angle, had to be be redone... a lotta wrk
(besides cost of the new hi $$ parts,
and time spent, NOT MAKING CAR go qwiker other ways on ur list..)
for a resultant of not running any Fookin qwiker. W/ the textbook, supposedly known quantity, lower drag pinion ht design.

Fook, dude.. Even Us dumbazz's, knew this wasnt gonna wrk, long ago, beforethe millionsgot spent.
We learned most of it, on the GM 12 bolt junk ( aka 8.8") ordeal, in 80s & 90s.

Thats why talking to the ole idiots (like us)
who cant hardly run Iphone to its capabilities...
Still know the history of there craft, & sometimes.. history can tell / help you predict the future.

When ur a bit lucky, & a bit open minded,
to hearing what ope guys learned bk in 1975... that did.. or didn't ... wrk..

If u still need more... info... I'll likely provide anything, thats 10yrs 'new technologically', or doesn't compromise our current, or past, 36 month clients program, that they paid fir.

Otherwise all this useless info,

(Of past learned, tech info, that only cost us a lifetime.. & 65,000++ hrs to learn)

as it ALL jus dies W/a man.... no pockets needed in last pair pants u Will EVER wear,
to take ur best secrets w/U, on bk of a napkin..

THERE'S ALWAYS a solution to a problem. Cost, amt of wrk Might not fit your wallet.

Example:
Put a rearend in ur car, that has a Ratio u want, thats easily available.
Jus 1 Example of a solution,
to a gear ratio problem availability problem.
Solutions are easy to dream of. Not always easto implement.

I gotta run...
& do something, even if its wrong!!
 

edco

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Yikes! This discussion is deep. I have learned things. IRS v solid axle; I own them both and like them both. I would not give up my '07 GT, I get too much pleasure from it. Kind of like that old time rock and roll. I have my SRA tuned to perfection. It is a work of automotive art. Just like the 4.6L 3 valve, yes they invented twenty-nine more powerful engines, but I have never driven the GT without smiling my ass off at some point.

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accel

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VW Bugs went to IRS in 1969. Now you know.
I was reading a book on Ford Mustang the other day. Little known fact was that it initially had a IRS option available which was discontinued due to low demand
 

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Well. Yeah. In an <8.8- Super situation> .....very few guys ask... % wise...
Bullitt, that's a mess to read, but reasonably helpful.

I think many OEM engineering decisions are much more about reaching fuel economy numbers instead of building a stronger or faster car. To your point, a Super 8.8 wasn't designed to transfer big power without breaking. It was designed to transmit small amounts with minimal loss. Using 24.8 HP instead of 25 to go 60mph is a big deal to an OEM because of governmental standards, not consumer demand.

It looks like I'll have to be happy with a 3.31 gear from Ford and the tallest tires I can find (which limits selection).
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